Each year, about 20 million tons (150 million barrels) of used lubricating oils, such as automotive lubricating oils, gear oils, turbine oils and hydraulic oils which through usage or handling have become unfit for their intended use, are generated world-wide. Used oil accumulates in thousands of service stations, repair shops and industrial plants, derived from millions of cars and other machines. Lubricating oil does not wear out during use, but does become contaminated with heavy metals, water, fuel, carbon particles and degraded additives. Eventually the lubricating oil is so contaminated that it can not satisfactorily perform its lubricating function and must therefore be replaced. Most of this used oil is dumped (legally or illegally) or burned as low-grade fuel, but such methods of disposal are highly detrimental to the environment and can cause serious pollution. Public opinion and governmental requirements are increasingly demanding the recycling, rather than the burning or dumping, of waste products. Used lubricating oil may contain 60 to 80% highly valuable base oil (generally comprising mineral oil fractions with a viscosity of not less than 20 cSt at 40 degrees Centigrade), worth significantly more than heavy fuel oil. It is therefore desirable to extract and reuse this base oil.
To date, however, recycling has not generally been undertaken by the refiners of crude oil. This is because, although used oil represents a sizable raw material source for re-refining, its volume is relatively small in relation to the world's crude oil requirements, which currently exceed 9 million tons (65 million barrels) a day. In addition, used oil is contaminated by impurities which can cause expensive disruption and downtime in conventional large crude oil refineries. Furthermore, since used oil does not generally originate from one source in large volumes, its collection and handling require resources which are incompatible with the normal raw material logistics of large oil companies.
It has been known since the early 1900s that used lubricating oil from engines and machinery can be recycled. Such recycling grew and developed with the popularization of the automobile. During the Second World War, re-refining became more widespread due to the difficulties in supplying virgin lubricating oil. Used oil re-refining still continued in the 1960s and 1970s, but then became uneconomical. This was because the conventional re-refining processes at that time involved the addition of sulphuric acid in order to separate the contaminants from the useful hydrocarbon components of the used oil, thereby generating as a waste product a highly toxic acid sludge. With the increased use of performance-enhancing oil additives towards the end of the 1970s, the amount of acid sludge generated by conventional re-refining plants grew to an unacceptable level. In the United States of America, it has been reported by the American Petroleum Institute that, as a consequence of legislation prohibiting the land filling of acid sludge generated by conventional re-refining operations, the number of used oil re-refining plants has dropped from 160 in the 1960s to only three today.
As an alternative to the acid treatment process for the re-refining of used oil, various evaporation/condensation processes have been proposed. In an attempt to obtain high operating efficiency, it is generally suggested that thin film evaporators be used. These evaporators include a rotating mechanism inside the evaporator vessel which creates a high turbulence and thereby reduces the residence time of feedstock oil in the evaporator. This is done in order to reduce coking, which is caused by cracking of the hydrocarbons due to impurities in the used oil. Cracking starts to occur when the temperature of the feedstock oil rises above 300 degrees Centigrade, worsening significantly above 360 to 370 degrees Centigrade. However, any coking which does occur will foul the rotating mechanism and other labyrinthine mechanisms such as the tube-type heat exchangers which are often found in thin film evaporators. These must therefore be cleaned regularly, which leads to considerable downtime owing to the intricate structure of the mechanisms.
It is known from WIPO Document Number WO-91/17804 dated November, 1991, to provide an evaporator which may be used in the re-refining of used oil by distillation. This evaporator comprises a cyclonic vacuum evaporator in which superheated liquid is injected tangentially into a partially evacuated and generally cylindrical vessel. The inside of the vessel is provided with a number of concentric cones stacked on top of one another which serve to provide a reflux action. As a result of coking, however, the evaporator still needs to be shut down periodically in order to undertake the intricate and time-consuming task of cleaning the cones.
U.S. Pat. No. 5,814,207 discloses an oil re-refining method and apparatus wherein a re-refining plant comprises two or more evaporators connected to one another in series. Feedstock used oil is first filtered to remove particles and contaminants above a predetermined size, for example 100 to 300 .mu.m, and is then passed to the first evaporator by way of a buffer vessel and a preheating tank, where the feedstock is heated to approximately 80 degrees Centigrade. Additional chemical additives, such as caustic soda and/or potash, may be introduced at this stage. The feedstock is then injected substantially tangentially into the first evaporator, in which the temperature and pressure conditions are preferably from 160 to 180 degrees Centigrade and 400 mbar vacuum to atmospheric pressure respectively. Under these conditions, water and light hydrocarbons (known as light ends, with properties similar to those of naphtha) are flashed off and condensed in the spray condenser of the evaporator and/or in an external after-condenser. These fractions generally account for between 5 to 15% of the used oil volume. The cyclonic vacuum evaporation process combined with the use of a spray condenser produces a distilled water which has a relatively low metal and other contaminant content. Light ends present in the water are then separated, and may be used as heating fuel for the re-refining process. The water may be treated in order to comply with environmental regulations and may be discharged or used as a coolant or heating fluid in the re-refining process. The bottoms product, comprising the non-distilled 85 to 95% of the used feedstock oil, is recirculated as described above. In the recirculation circuit, the bottoms product is heated, preferably to 180 to 200 degrees Centigrade, and mixed with the primary feedstock supply for reinjection into the first evaporator. Advantageously, the pump in the recirculation circuit generates a recirculation flow rate greater than the initial feedstock flow rate. This helps to reduce coking in the recirculation pipes since overheating of the oil in the heat exchanger is avoided. The recirculation flow rate should be large enough to generate a well turbulent flow, and accordingly depends on the heat exchanger duty and on the size of the pipe lines. This is typically achieved with a recirculation flow rate 5 to 10 times greater than the initial feedstock flow rate.
A proportion of the recirculating bottoms product from the first evaporator is fed to and injected into a second evaporator. This second evaporator is substantially similar to the first evaporator, but the temperature and pressure conditions are preferably from 260 to 290 degrees Centigrade and 40 to 100 mbar vacuum respectively. Under these conditions, a light fuel oil (similar to atmospheric gas oil) and a spindle oil (having a viscosity at 40 degrees Centigrade of about 15 cSt) are flashed off as overhead products, leaving behind a bottoms product from which the base oil distillate is to be recovered. These gas oil and spindle oil fractions generally account for between 6 to 20% of the original used oil volume. The condensed fractions are fed to storage and may be subjected to a finishing treatment, the severity of which will be determined by final usage and market requirements. The bottoms product of the second evaporator is recirculated as in the first evaporator, but at a temperature preferably in the region of 280 degrees Centigrade, and a proportion of the recirculated product is fed to and injected into a third evaporator.
The third evaporator preferably operates at temperature and pressure conditions of around 290 to 330 degrees Centigrade and 15 to 25 mbar vacuum respectively. These operating values may be varied within predetermined limits (generally +/-10%) to suit the required distillate output products. Advantageously, the third evaporator is in communication with first and second spray condensers. The second spray condenser serves to condense some of the lighter fractions from the vapor phase which passes through the first spray condenser.
Two base oil fractions are produced in the third stage as overhead distillate products and fed to storage. The first and second spray condensers, operating at elevated temperatures (100 to 250 degrees Centigrade) allow a partial condensation whereby two specific distillate fractions can be produced. The spray condensers have the added advantage that the temperature as well as the recirculation flow rate can be varied, thereby allowing a flexible fractionation. The viscosity of the fractions may be altered by adjusting the ratio of temperature to recirculation flow rate; by increasing the condenser temperature, a heavier oil fraction can be produced. The base oil fractions extracted by the third evaporator generally account for about 10 to 50% of the used oil volume. The bottoms product is recirculated at around 330 degrees Centigrade as before, and a proportion of the recirculated product is fed to and injected into a fourth evaporator.
The fourth evaporator preferably operates at temperature and pressure conditions of around 320 to 345 degrees Centigrade and 5 to 15 mbar vacuum respectively. Further base oil fractions, which are heavier than those extracted in the third stage, are flashed off as overhead products and are condensed as base oil distillate fractions and fed to storage. In certain embodiments, the evaporator may be operated in a blocked manner, whereby a number of discrete temperature and pressure conditions are applied in order to extract specific fractions from the feedstock. Each such fraction is preferably fed to individual storage. The base oil fractions extracted by the fourth evaporator generally account for about 10 to 50% of the original used oil volume; this depends to some extent on the general viscosity of the used feedstock oil. The remaining bottoms concentrate contains heavy metals from the used oil, and sediments, carbon particles, ash and various non-volatile oil additives. This bottoms concentrate is fed to storage and is suitable for use as a roofing flux, a cold patch material or an asphalt extender. Where environmental regulations permit, the bottoms concentrate may be used as a heavy fuel oil in applications such as cement kilns, blast furnaces or incinerators. Dependent on its intended usage, the evaporator conditions may be set to produce a bottoms concentrate at viscosities ranging from 380 cSt at 40 degrees Centigrade for heavy fuel to 200 cSt at 135 degrees Centigrade for asphalt use.
The distillate fractions typically amount to 85-95% of the used lubricating oil, leaving 5-15% as bottoms. The base oil distillate fractions may be treated to produce finished base oils (which have viscosities of not less than 20 cSt at 40 degrees Centigrade and have characteristics similar to those of virgin base oils). Depending on the fractions contained in the used oil and on market requirements, the base oil fractions that are typically produced are 100 SN (solvent neutral), 150 SN, 250 SN and 350+SN. If only one or two wider base oil fractions are required, the fourth evaporator may be omitted.
As an alternative to the multi-stage distillation plant described above, it is possible to utilize a single evaporator operating in a blocked manner. The various fractions may then be extracted sequentially by applying predetermined temperature and pressure conditions in the evaporator. This has the advantage over a multi-stage plant of requiring less capital expenditure, but is less efficient since continuous process conditions can not be achieved.
The raw base oil distillates may contain volatile contaminants, oxidation compounds, unstable sulphur compounds and various decomposition products from additives, depending on the type and quality of the feedstock. It is therefore advantageous to provide a finishing treatment in which base and fuel oil distillates are chemically treated in order to remove unstable or other undesirable components.
Copending application Ser. No. 09/250,741, filed Feb. 16, 1999 assigned to the assignee hereof discloses a method of removing acidic compounds, color, and polynuclear aromatic hydrocarbons (PAHs), and removing or substituting heteroatoms from used oil distillates, such as those produced by the foregoing process. In the practice of the method, an organic or inorganic base, a transfer catalyst, and the used oil distillate are mixed and heated. Thereafter, the contaminants are removed by distillation. The method may be operated either in a batch mode or in a continuous mode. When the continuous mode is used, the method may be used prior to, or concurrent with, the method of U.S. Pat. No. 5,814,207 as described above. By means of the method, the complexity of the apparatus of the '207 Patent is substantially reduced.
PAHs are a frequently found class of contaminants in used motor oils, especially, used oils generated from Diesel engines. PAHs are found in virgin motor oils, albeit at low levels. PAHs are more concentrated in used oil as PAHs are produced in the combustion process that takes place in gasoline or diesel fueled engines.
As some PAHs are suspected carcinogens, it is desirable to remove the PAHs from used motor oil to enhance the value and quality of re-refined motor oils. In addition to PAHs, other contaminants exist in used oil that are difficult at best to remove through distillation or chemical treatment. These compounds include sulfur and nitrogen-containing organic compounds and compounds that absorb light which leads to a colored appearance of the re-refined oil.
Traditionally, PAHs have been removed from used motor oils through hydrotreating. Hydrotreating is a hydrogenation technology by which a used oil distillate is exposed to high pressure hydrogen and a catalyst at a high temperature. The resulting oil is typically lower in PAH content and other contaminants. While somewhat effective, hydrotreating is extremely expensive, so much so that it is frequently not economically feasible as a used oil re-refining process. Additionally, a major drawback to hydrotreating is the fact that the products resulting from the hydrotreating process remain in the used oil. These compounds may, at times, be more mutagenic or carcinogenic than the original PAH molecules.
The process of the above-referenced copending application is successful in removing PAHs from used motor oil to a certain extent. In many instances the results obtained by the method of the copending application are quite adequate. It has been found, however, that PAH's, sulfur-containing substances, nitrogen-containing substances, and other contaminant remain in the used motor oil after it has been processed in accordance with the method of the copending application. The present invention comprises a process which is employed after the method of the copending application to further reduce the presence of PAHs, sulphur and nitrogen-containing substances, and other contaminants from used motor oil distillates.
The present invention is especially applicable to the removal of contaminants from used oil distillates. The invention is also useful in removing PAHS, sulfur-contain substances, nitrogen-containing substances and other contaminants from virgin oil distillates and other petroleum distillates, it being understood that in most cases virgin oil distillates and similar petroleum distillates will not require pre-processing in accordance with the method of the copending application. Other applications of the invention will readily suggest themselves to those skilled in the art.
In accordance with the present invention, petroleum distillate is contacted with a highly polar organic solvent, such as N, N-dimethylformamide (DMF). It has been found that DMF is especially selective towards PAHs. Additionally, it has been found that in addition to PAHs, the solvent system is also selective towards various sulfur-containing molecules. Sulfur-containing molecules are undesirable in base oil and other petroleum products as they decrease the overall oxidation stability of the petroleum products.
While solvent extraction is a well known technique for manufacturing virgin base oil, its use in the manufacture of re-refined base oil is not well known, if at all. Further, the solvents used in the manufacture of virgin base oil are less polar than the solvents used in the present system. The lower polarity of the solvents commonly used in base oil manufacture leads to a significant loss of desirable base petroleum compounds.
Specifically, the present invention consists of a liquid/liquid extraction system in which the petroleum distillate is contacted with the organic solvent. As the organic solvent is imiscible with the petroleum distillate, the recovered solvent is easily separated from the petroleum distillate after the appropriate contact. Any residual solvent in the petroleum distillate is easily removed through evaporation, adsorption or other common separation methods. The spent solvent is easily separated from the extracted PAHs and other contaminants, and can be continuously regenerated and used.